Hello,
Like I said before I know I ask a lot of dumb questions, however I had an idea and was wondering if it is possible.
I would like to wire two 8ohm speakers to an amp. Is there any way to put a split second delay on one
of the speakers, so that it is transmitting the same signal only a split second later? I don't know if it is at all
possible, or how much trouble it would be even if it could be done.
Put one close to the amp, the other 30/90 ft away.
There's your delay.
I imagine it could be done with some sort of circuitry between one speaker and the other to delay the signal but I wouldn't know what that would be. Might be easy, might not. If you do it, I'd rig it so you can bypass it with a switch. I doubt you want permanent delay/echo effect on your amp. But whatever turns you on is what matters. :dbtu:
The only dumb question is the one you fail to ask.
Apart from a very long speaker lead to a box in the parking lot, ::) the normal way to do this would be some sort of "low level" delay device - a delay pedal, MIDIverb, etc., and a second amp to drive the delayed speaker. There is no simple, or even complex, way to insert a significant delay at what we call "high level", i.e. between amp output and speaker.
Hello,
The delay was just going to be another way to try to get the amp to sound a little more tube. It is not necessary.
My main objective was to get the amp to sound as close to a Fender Champ as possible, without all of the constant tube changes, biasing, and all of the other upkeep required maintaining a tube amp. On some of the other sites it seems like people get obsessed with just that. I just want reasonable sound you can just turn on and turn off, without rebuilding the amp on each occasion.
I do not use pedals and such. I have a Squier Telecaster, and an Epiphone Les Paul JR. I like to just run them straight into the amp. Those guitars are very basic and crude, and so was the Fender Champ.
I have 18 other guitars, and a Crate GX-40C+ with chorus and reverb. I just really like the raw gritty sound of low tech instruments and electronics from the 50's and 60's.
The diode mod and the 12" speaker really got it in the ballpark, I am well pleased with both those changes.
:)
Quote from: J M Fahey on December 24, 2013, 04:34:04 PM
Put one close to the amp, the other 30/90 ft away.
There's your delay.
Hello,
Sounds good, by the way could I mail one to you with a remote transmitter attached to get a really good delay ?
Please do experiment as much as you can to learn, but always remember, this simply is not a Champ. Imagine a restaurant that sells lobster. And a chef trying thing after thing to make his lobster taste more like steak.
Hello,
OK, Enzo I understand. As good as it sounds now will just have to be good enough considering what it is.
Thank you.
Hello,
Is this something to work with ? Someone on another forum said it was capable of being converted to a 5e3. At least it is tubes and some of the same components. Since it is tubes should I post about it on another forum.
Generally speaking the problem with converting a s.s. amp to valves isn't so much electronic as physical. If you look at a few amps of each type you start to notice that s.s amps tend to be built on one side of a U-shaped chassis while valve amps tend to be built on both sides of an inverted U-shaped chassis.
It's not that the problems of the physical layout can't be overcome, but after a bit of head scratching you generally come to the conclusion that it would be easier in the long run to make a whole fresh start with a case and chassis arrangement intended for valves and build your valve amp on that.
From what I can make out of your Newcomb circuit it's for something like a factory PA and I'd guess from the valve lineup and drafting dates from the late 50's to early 60's. Even modified (and it would require a fair bit of modification) I don't think you would find this a very satisfactory guitar amp.
Hello Roly,
I am not talking about converting the SS to tubes. I am wondering if the Newcomb would be a better choice for modification. I have ran a guitar through it and it is loud. It seems to have to much gain. I can understand
why as it was probably more for talking as opposed to amplification of music. I am going to make a cabinet as
I have one 12" 16ohm speaker. I will just see how it works. I am not disregarding your advice, I know it will be
a problem. I appreciate all of your input and all of the advice you have given me in the last few weeks. Thank
you very much.
Ah, sorry, I misunderstood; you already have one of these and want to modify it.
Well as it happens there are a line of AWA/STC (http://www.ozvalveamps.org/awa.htm) factory PA's that were made here ranging from a few watts up to 120W which are very similar and many of which have been pressed into guitar service. Tim Robbins is a great bloke who has done many like this and you can contact him here http://dalmura.com.au/projects/amplifier.php (http://dalmura.com.au/projects/amplifier.php).
The circuit is very small and blurry (or maybe it's me that's blurry :( ) but an important point is what is available on the output transformer secondary. Many of these were made to drive "voltage line" rather than low impedance speakers, but it looks like this one might have suitable low impedance tappings. If not there are often suitable sections of the winding that can be found between tappings.
While it may seem to have too much gain now, particularly if you go in via the mic input, you will want to fit some sort of tonestack, the Big Muff being a single-knob improvement on the simple top-cut fitted, and this will have about 20dB loss so the extra gain will tend to be compensated that way.
The input transformer for the microphone won't be required, but put it to one side because these can be useful to provide an isolated line out for recording etc. It will have a mu-metal shell that should be treated like glass.
The Phono input should have about the right sensitivity for clean guitar, but you can press the mic preamp into service if you want a "crunch"/gain stage, but the component values will need to be changed to something a bit more conventional, say 1Meg on the grid, 100k on the anode, and about 1k2 on the cathode, bypassed to taste with around 25uF.
The input coupling cap is a pretty small value and may need to be fattened up to say 0.1uF, and taken direct to the input jack tip. Ditto with the other coupling caps, but then I expect that these will be the dreaded waxed paper types which will now be leaky and require replacement anyway. The interstage caps need to be HT rated.
Three core mains lead with a secure ground to the chassis.
You can experiment with opening the 180k negative feedback resistor path for possibly a bit more "tone" (i.e. distortion).
The 100k pot in the middle is to balance the drive to each of the output valves (it's a slightly unconventional Phase Inverter, but no matter if it's working).
Naturally you will have to evaluate the health of the electro caps on the HT line which must be about 50 years old by now, the rectifier (which may be replaced by silicon diodes), and the 6V6's for output. Until you are certain of the electro caps use a limiting lamp.
Go over all the resistors too, and replace any that are more than 10% off value with one end disconnected.
But yeah, overall it's a very plausible project. Should give a very respectable 14-odd watts with healthy 6V6's.
HTH, and keep us in touch.
Hello,
Roly, I have replaced all of the caps just because of the age of the unit. I turned it on and it was so quiet I thought I had left something disconnected. I brought this one to our local amp repair person for him to check out. He told me he can only go through circuits and find problems, and that he is not an engineer. He does a wonderful job of troubleshooting and repairing problems, however he cannot discuss theory and tell you what will happen if you do or don't do this or that. Being that he is the only person knowledgeable in repairing tube type stuff in this area it is quiet frustrating at times. The only other option is going to the big city to have one of the people there tell you they don't know either.
I am going to pull it apart again and look at it. I will likely make some of the changes you suggested. At least this one works I just have to tone it down a bit. Sorry about the schematic. I pulled it off the web
because while there is one inside the amp I would have to take a picture of it, download it from the camera
upload it to the computer and then post it in this forum. That could take most of the day and may not be as good as the one you are looking at now. I was actually surprised the schematic popped right up on the web.
Anyway thank you for all of your help so far. I am sure I will have more questions soon.
Okay, assuming that you have something like an 8 ohm tapping on the output transformer and you have a good speaker connected to it, and you can power it up via a limiting lamp without smoke...
Are the valves lighting up?
Have you got 2-300V of HT coming out of the rectifier? Is it getting to the OPT centre tap? Is it getting to the 6V6 anodes? (you will need the data sheet to identify the valve pins)
How much voltage is across the common cathode resistor after the valves have warmed up? (this tells us how much current is, or isn't, flowing in the output pair).
When you probe the 6V6 grid pins is there any sign of life from the speaker, faint scratches or pops?
If so, what are the voltages on the two Phase Inverter valves, anodes and cathodes? Do you get (louder) scratchies when you probe either PI grid?
I won't take it further at this point, but the idea is to first make sure the voltage supplies are sufficient, then starting from the output working backwards along the signal chain looking for signs of life from the speaker at each stage, sorta like finding a blockage in a pipe.
{By the time you're done with this he'll be bringing problems to you}
HTH
Hello,
Roly, I don't think I have to go through all of that. I worked on it in the last few days. It is coming along well. I have a Fender Tweed Deluxe tone stack in it. I don,t know if that is the best one but I found a schematic for it, it may need changed or modified at some point. Would the volume need a 470K resistor in front of it to load the guitar pickup ? Some guitar amps have a .68-100K in that position.
I changed the grid resistor from 18K to 1.4K that was the lowest value I had. I also added another resistor in parallel to the 560K plate resistor 510K, it should be roughly half now.
I fired it up today and it sounds much better. Some of the gain is gone, however it is still leans a bit towards the bass side. I believe I can work on the caps to iron that out. I also think a better speaker will be in order when I get it operational.
So I am still just swimming around in the dark, but its getting better. Thank you once again.
Good to hear.Tonestacks are a rich and wide open area for the amp tinkerer. Most of them have an insertion loss of around 20dB so they are pretty interchangeable, the most common limiting factor being the number of holes in the front panel (heh heh).
Quote from: SpareRibsI changed the grid resistor from 18K to 1.4K that was the lowest value I had. I also added another resistor in parallel to the 560K plate resistor 510K, it should be roughly half now.
I hope you mean "cathode" resistor down to 1k4.
As original the Mic input stage uses "contact" bias and will be overdriven by a guitar signal, so it needs to be modified to use more conventional cathode bias.
You will find a table of typical values for use with a 12AX7 here (http://www.ozvalveamps.org/rcatriodes.htm).
The two most common in guitar amps are;
100k anode load and a bypassed 1k5 in the cathode for a gain of 52, and
220k anode load and a bypassed 2k2 in the cathode for a gain of 59.
While these are for a nominal 300V supply they should be okay down to at least 200V. The main tweek is the cathode resistor value to get the resting anode voltage somewhere around 1/2 to 2/3rds of the stage HT supply.
Normally the cathode resistor would be bypassed by a low voltage 25uF cap for full bandwidth gain, but as you are finding it a bit bassy a tweek is to reduce the value of this cap. In some amps it is reduced very considerably to make a "lead" channel, perhaps as low as 0.1uF, but feel free to experiment to taste. The cathode voltage is normally only a couple of volts.
The grid resistor should be reduce to between 1Meg and 2.7Meg (see below).
Assuming you are currently connecting your guitar to the "Phono" input; this has a 1meg pot which will define the input impedance up to a setting of 7 or 8/10 and this should be satisfactory for just about any guitar.
I have done some experimenting with amp input impedances (resistances) and found that one meg should be the
lowest you should go with a
passive guitar (one without a battery inside), going lower tends to make the pickup dull, and that increasing the input resistance to 2 or 3 megs can give and improvement by unloading the pickups and making them more "lively". You could try inserting a resistor, say half to one meg,
between the Phone input and the volume pot, i.e. in
series with the input, not across it. This will reduce the signal level by half and present the guitar with a high input resistance. You will find a detailed discussion here (http://www.ozvalveamps.org/pickups.htm).
This does not apply to guitars with active pickups, those powered by an internal 9V battery which are already unloaded by an internal preamp/buffer.
HTH
Hello,
Yes Roly I did indeed mean the cathode resistor. I used the jack on the mike input but I bypassed the thing that looks like a tube socket. I wired a 68K resistor, a .01 cap both in line directly into #2 on the 12AX7. Coming out the #1, the (Fender Tweed Deluxe) tone stack into #1 on the 6AU6. I used the existing potentiometers. I also left the 330K resistor on the tone control wiper I think it may have remove that to let the signal pass into the 6AU6 with less resistance.
I got all of that done, it still sounds a bit tubby. Tomorrow I am going to try different values of cathode bypass capacitors, currently there is a 25uf - 25v. I am going to try 10, 5, and 1uf. I think I will start at 1 and work up, as I already know 25 is to much.
It is going well. I think most of you guys could have done this in an afternoon, however I have gone
this far without any clouds of smoke
Since you have moved away from the original circuit may I suggest that you at least draw and post the bit that you are modifying so we can better follow what you are doing? TIA
Hello,
The Fender Tweed Deluxe tone and volume circuit is in (Converting An Existing Amp Chassis For Guitar Use by Niels"Petco"Nielson) Pg.7. I put it between the first half of the 12AX7 pin #1 and pin #1 on the 6AU6, also I properly grounded it.
I could not find a way to bring both drawings together. I was wondering why you would want it posted, but now I can see there are no two alike.
I would also like to ask could that treble circuit be eliminated ? It would only constitute cutting one wire at pin #5 on the 6AU6. It seems like it is out of the circuit and only makes obnoxious noise.
I hope this helps. Thank you for your continued intrest.
Typical triodes
http://www.ozvalveamps.org/rcatriodes.htm (http://www.ozvalveamps.org/rcatriodes.htm)
...more...
http://www.ozvalveamps.org/ampstages.htm#inputstage (http://www.ozvalveamps.org/ampstages.htm#inputstage)
Typical triode guitar input stage
(http://www.ozvalveamps.org/generic/stagesinpre1.gif)
Fenderish tonestack and gain recovery post amp
(http://www.ozvalveamps.org/generic/stagestonevol.jpg)
Both the pot and the cap in the top cut has to be suspect, and it's not what we want anyway. The idea is to "tune" or tweek the bandpass and controls to be more guitar amp-like. Fresh hardware.
A typical limitation at this point with small PA's is that there is only one hole for a tone control, so the Big Muff single knob is worth a look (In Tone Stack Calc).
Hello,
I am going to incorporate your second drawing into the schematic and post it back later today.The PA has three holes in the front, another one could be utilized if I dumped the power on light. I don't know a lot about uploading and downloading things, but I will struggle through it .
I didn't think the Fender tone stack would be sufficient as a Fender Deluxe is kind of a crude amp to start with. I actually wanted a tone stack with mid range but I didn't know where to look. However I will worry about that later. The first thing is to iron the bugs out of this.
Thank you once again for your time and effort.
Hello,
I hope this works. (GREAT), are we on the same page ? I think I put it in the right position. I had never thought of putting the tone stack in front of the 12AX7, Duh !!!! Maybe that's why they call it a preamp.
I made one mistake, on the cathode resistor of the 12AX7 I marked it 1.5K, but I know it is 1500 as per the schematic. Thank you for the diagram. I am going to start on it right away.
1k5 == 1500 ohms!
Input -> 12AX7a-> (Gain)-> tonestack-> 12AX7b-> Master Vol-> PI-> OP->
The attached circuit is unfinished, potentially full of value errors because the original was so lo-def; hasn't run yet in LTSpice because the OPT is still unknown - we need to know the inductance and resistance of each winding section.
It also already departs from the original because I didn't see a lot of point in redrafting it when we have a rough copy and a sample of the beast, so it's what I'd be thinking of doing (remembering that the PI may still have a couple of "issues").
A bunch of voltage measurements, supplies, anodes and cathodes, warm idle and normal programme drive, would be a great help.
Also...
http://www.r-type.org/pdfs/6au6-1.pdf (http://www.r-type.org/pdfs/6au6-1.pdf)
Hello Roly,
I have spent a lot of time pondering over the diagram you posted. I do not know how to check all of the voltage measurements of supplies, warm idle, or normal program drive. I intend to incorporate all of the changes to the left of R6.
I have one question at this point. Coming off the plate of the 6AU6, (on the original schematic) there is a resistor immediately to the right, a 560K that ties the 6AU6 plate to the plate of the second half of the 12AX7. Is that connection to be eliminated ? As on the drawing you posted it is not there. I want to be sure before I start tearing things out. I will have both schematics and make notations as I go along. So in the
worst case scenario I will know how things were before I started
Thank you for the effort you expended in creating the drawing. Also in case no one else told you yet,
Happy New Year !!!!!
Appended cct 0v2 annotated showing:
Red - main HT feed to output stage
Yellow - screen HT feed to OP stage
Orange - HT to PI and preamp
Green - signal flow
Master vol-> 6AU6 -v----------> 6V6a
'-> 12AX7-> 6V6b
The output of the 6AU6 is attenuated by R4 and R5 and then inverted by the 12AX7 stage, the drive balance set by the attenuator trim, R6.
Can you go over this cct for values errors, and tell me what you actually found in the chassis, TIA.
Hello,
Yes Roly, I will post those readings as soon as I finish cobbling it together. I should have it done sometime
tomorrow. I will also give you the readings of the 6AU6.
Should the guitar be plugged in, or will that matter ?
Normally initial standard test conditions are with the input shorted. In most guitar amps, but not all, the input jack socket already takes care of this, otherwise it's a clip lead or shorted plug.
For signal tests something like a cassette player or MP3/WAV player will serve as a satisfactory signal generator, particularly if you have pre recorded a number of test tones and sweeps in something like Audacity.
Tone tests with a guitar follow when you have the mechanics sorted, then you get down to fine tuning/tweeking.
First we apply power for a smoke test.
Then we look at the DC conditions of each stage, starting with the output pair, and working back to the input.
HT1,2,3, every anode and cathode voltage from output to input.
Then we take Berlin.
Hello,
OK, I'm on it will post results later today.
Hello,
I spent most of the day scratching my head and soldering. I am almost done. I have to make another hole in the front panel for the gain control. That is going to regulate the value of the 25uf bypass cap correct ? I
removed the pot for the treble control (original schematic) was stacked on top of an on and off switch. I just
removed it since that treble circuit was going to go anyway, and replaced the rotary on and off switch with a toggle switch
Now I have the on and off toggle, bass, treble, and master volume. That makes it cleaner and the pots are the proper value. When I get the other hole for the gain everything will be arranged in better order.
OK as soon as I complete all of this tomorrow I will do the readings. I thought I was going to get it all done today (Ha Ha Ha) !!!!
Thank you for all your help so far. I could have never imagined doing all of this on my own. I didn't know
where to start or what to do.
:)
Keep the switchpot. Goes in your junk stash of potentially useful parts.
Before you go drilling any holes or adding a Gain pot, first just connect the -ve end of the cathode bypass to ground and do some initial tests, we'll add complications in small steps if you don't mind.
Limiter bulb (wattage?, supply voltage?)
VHT1,2,3 =?
We need table of Va and Vk for all valves.
6V6a Va, Vs, Vk
6V6b Va, Vs, Vk
6AU6 Va, Vs, Vk
12AX7a Va, Vk
12AX7b Va, Vk
Hello,
I used a light bulb limiter with 60 watt bulb. Light came on bright then went to nothing. Second try light came on dim also went to nothing. I unhooked the speaker changed to 25 watt bulb now I have maybe half the
output of the bulb constant glow.
I checked the voltage on the 6V6's it was 95v with light bulb. Is it ready to plug directly into the wall ? Or should I take more readings first ?
Tube amps have intrinsic high power consumption, even at idle, so the 25W bulb is definitely too small, and probably the 60W one too.
Use at least a 100W one and if everything looks normal, plug it straight into the wall.
It should go quite dim after a bright start, 60W or 100W.
Then post...
Quote
VHT1, 2, 3 = ?
We also need table of Va and Vk for all valves.
6V6a Va, Vs, Vk
6V6b Va, Vs, Vk
6AU6 Va, Vs, Vk
12AX7a Va, Vk
12AX7b Va, Vk
Hello,
With limiter still on the voltages are as follows. I only have voltages as I am not sure what Va,Vs,or Vk,are.
1.6AU6 pin 1= (-2) 1. 12AX7 = Anode pin 1 (286) / Cathode pin 3 (259)
2.6AU6 pin 5=(293) 2. 6AU6 = Anode pin 5 (282) / Cathode pin 7 (0)
3.C3 = (293) 3. U1-6V6 = Anode pin 3 (265) / Cathode pin 8 (15)
4.R15 = (293) 4. U2-6V6 = Anode pin 3 (265) / Cathode pin 8 (16)
5.12AX7 pin 6 = (61) 5. 6X4 = Anode pin 6-1 (fluctuates 1-4) / Cathode (295)
6.12AX7 pin 7 = (0)
7.6V6 pin 5 = (0) both tubes
8.6V6 pin 4 = (284) both tubes
Removed resistor 560K right of pin 5 6AU6 going to pin 6 of second half of 12AX7 on original schematic
as it was missing from appended cct.
Is it now ready to plug directly into the wall ?
Quote from: SpareRibsIs it now ready to plug directly into the wall ?
NO!We have a way to go yet, and the supply voltage is only down 5% on spec; from an initial glance at your voltages you worked it out, Va = voltage anode, Vk = voltage kathode (don't ask), Vs = voltage screen, which is what you've done.
I'm going to do some LTSpice stuff with those numbers and see what that shows up. Stand-by.
Meanwhile...
What do the voltages around the first triode, 12AX7a, tell you, (259) volts on the cathode for example?
You've got 15V on one 6V6 cathode and 16V on the other, what do you make of this?
What is the actual marked and measured value of the 6V6 cathode resistor?
Hello,
Glad to know things are somewhat normal. I will do all of the readings over again using alligator clips. The
way I took the readings I posted was using an alligator clip for ground and a probe to get the readings, that may have led to the one volt difference. I will check them ALL again just to be sure.
I will also be sure to do the 6V6 cathode resistor.
Hello,
OK, here are the resistor readings using alligator clip. Some are slightly different but should be in spec.
Master wiper = (234) 12AX7 = PIN 1 = (294) 6AU6 = PIN 5 = ANODE (291)
C3 = (295) PIN 2 = (20) PIN 7 = CATHODE (0)
R15 = (295) PIN 3 = (270)
6AU6 = PIN 5 = (291) PIN 4 = (0) U1 6V6 = PIN 3 ANODE (261-269)*
6AU6 = PIN 1 = (-2) PIN 5 = (0) U1 6V6 = PIN 8 CATHODE (16)
12AX7 = PIN 6 = (295) PIN 6 = (295) U2 6V6 = PIN 3 ANODE (271-275)*
12AX7 = PIN 7 = (0) PIN 7 = (0) U2 6V6 = PIN 8 CATHODE (16)
6V6 = PIN 5 (0) Both tubes PIN 8 = (271)
6V6 = PIN 4 (285) Both tubes PIN 9 = (0) 6V6 CATHODE RESISTOR 16V ACROSS
VALUE = 300-OHMS
COLOR -ORANGE-BLACK-BROWN-GOLD
*Indicates fluctuation between two values
Hope this helps. Alligator clip was a tight squeeze in some places. One more question. I am working on this
out of the cabinet. Should the speaker be hooked up ? It is of great concern to me as it is said not to power it up with the speaker disconnected. However I am not putting under a load, but I am thinking it should be there to complete the circuit.
{I actually asked you those questions to get you to think about them, and what they might imply.}
With your latest readings a couple pop out at me.
Quote from: SpareRibsMaster wiper = (234)
Yawot? You've got
234V on the
wiper of the Master pot?
Really? Take a look at the circuit - how can this be?
What do you read on the other two lugs, particularly the nominally "grounded" lug?
Are there any voltages on the tonestack pots?
Secondly, the 12AX7, both sections
Quote from: SpareRibs
PIN 1 = (294)
PIN 2 = (20)
PIN 3 = (270)
The
cathode is at
270 volts? Really? Look at the circuit - there's (supposed to be) a 1k5 cathode resistor to ground.
Apply Ohm's Law, how much current is that?
Apply the Power Law, how many watts is that in the cathode resistor?
If it isn't cooking then my best guess is that it's open circuit somewhere.
(pins 4, 5, and 9 are part of the heater circuit and normally not quoted unless there is a specific heater problem, then in AC volts which they typically are.)
Now the other section...
Quote from: SpareRibs
PIN 6 = (295)
PIN 7 = (0)
PIN 8 = (271)
Again, we are supposed to have a 1k5 cathode bias resistor to ground, but we clearly don't.
What we want is for
the anode voltage to be about half the supply, and to get that we need to pass a bit over 1mA though the 100k anode resistor, and therefore also through the 1k5 cathode resistor, which should then be creating a drop of around 1.5 -> 2 volts on the cathode - ideally. Both sections.
So you need to find out why the 12AX7 stage (and maybe the Master pot) doesn't seem to have a ground return to the power supply.
An open output shouldn't be a problem but if you are worried about it just put a clip lead or tack a link across the output to short it, or a 8 ohm 10W w.w. resistor if you happen to have one to hand. ;)
Hello Roly,
Well I found the grounding problem. A is strip riveted and soldered to the chassis. It has 3connections.
I thought they were all grounds, I was wrong. Only the middle is ground. so I soldered the grounds to that. I now have some new numbers and they are radically different. So here goes.
C1 = 296
C2 = 289
C3 = 226
R15 = (55)
12AX7 = ANODE (67) CATHODE (0)
6AU6 = ANODE (67) CATHODE (0) PIN 1 (.718)
6V6 = U1-ANODE (289) CATHODE (16)
6V6 = U2-" " " "
Master Volume (0)
I also had to change a couple of wires on the tone controls. As I was working on it I was looking at it backwards. As I said before to most of you guys on this forum, this is all second nature to you. I am not as well versed in electronics. I am sorry if you get impatient with me.
Whoops. That'll do it every time. The new voltages look more like it.
Quote from: SpareRibsAs I was working on it I was looking at it backwards. As I said before to most of you guys on this forum, this is all second nature to you. I am not as well versed in electronics. I am sorry if you get impatient with me.
Not so. I've been doing this stuff most of my life and I frequently have to double check things like pot wiring to make sure I've got it right.
Quote from: SpareRibs12AX7 = ANODE (67) CATHODE (0)
I assume that this is the Phase Inverter section. I find the cathode = 0 a bit curious.
On the supply end of the anode load R10/270k we have around 230 volts, and on the anode end we have around 70 volts, meaning that there is 230 - 70 = 160 volts across the resistor. The current through this resistor is then given by Ohms Law as;
I = E / R
160 / 270 = 0.5925925mA
If this current is going through the valve section, as we would hope, then it must also be going through the cathode resistor, R11/1k5. Again Ohm's Law gives the expected cathode voltage;
E = I * R
0.59 * 1.5 = 0.885 volts. (On high voltage ranges your DMM will show shuch a small voltage as "0", so you'll need to use a lower voltage range for measuring cathode voltages).
BTW, because the resistance of the grid circuit is typically many times the resistance of your multimeter there isn't much point in measuing grid voltage (except in the output stage) because the meter will so seriously load the grid circuit as to make the reading meaningless.
I'll leave it to you to use the same method to work out what the cathode voltage of the 6AU6 should be, but it will also be (very) small.
You don't give the voltage of the preamp section of the 12AX7, but I'll make a couple of guess-timates;
I
guess the valve current at 1mA
Therefore the anode voltage will be (1mA * 100k = 100V drop), 230 - 100 = 130 volts.
1mA through 1k5 gives 1.5 volts cathode bias.
Getting there. :tu:
Hello,
Thanks, it certainly feels good to be making progress. I guess that grounding strip was causing most of my
problems all along. No matter what I tried I was fighting an open circuit. Without knowing where or what to check I was just beating my head against a wall. Oh well onward and upward.
I will check the readings once again. There is a location on the meter for lesser readings. I will utilize it for the cathode reading, and post it back.
I set the meter on the more sensitive setting and here is the result, (12AX7) Cathode (0.532). It may not be as accurate as it should be as the meter is not high dollar,
90.532
Hello,
I am not sure I did this right, but here goes.
6AU6 Supply side R7 68 - Anode side 67
68-67= 1
1/68=0.014
Actual meter reading - 0.010
Preamp voltage - Anode 68 - Cathode .526
With audio electronics 10% accuracy is fine. For all the crud that goes on in Hyper-Fi forums about parts-in-a-billion effects, audio ain't rocket surgery, and guitar audio even less so.
Quote from: SpareRibs6AU6 Supply side R7 68
Really? But isn't this supposed to be HT3 @ 230-odd volts?
Now the anode load is 680k and the screen feed resistor is 1.9Meg, and I'm going to introduce you to something very important.
Pull the 6AU6 out of its socket, power up, and confirm the supply voltage (~230V), then measure the voltage on the valve end of the 680k anode resistor, then the valve end of the 1.9Meg resistor.
With the valve out there should be no current flowing in these resistors and therefore no voltage drop across them, right?
Is that what your measurements say?
Why do you think you are getting the result you are getting?
(yes, there will be a test next period. ;) )
Quote from: SpareRibsPreamp voltage - Anode 68 - Cathode .526
Okay, some harmless sums;
0.526V on the cathode, and 1k5 cathode resistor, therefore the current is;
I = E/R
0.526/1.5 = 0.35..mA
The same current must be flowing in the anode resistor, R15/100k, off the 230V supply, so does this add up?
Voltage drop;
E = I * R
0.526 * 100 = 52.6V (mA * K-ohm = Volts)
230 - 52.6 = 177.4V on the anode. But you measure only 68V, so that obviously
doesn't add up. In Ye Olde Days of analogue meters I'd say you mis-read your meter, but in these days of digital accuracy I'll say that you weren't measuring what you thought you were measuring (wrong side of the twin triode perhaps, confusing the preamp with the phase inverter maybe?)
On the
preamp section I expect around 1.5V on the cathode and around 130V on the anode (for a 230V supply).
Quote from: Roly on January 01, 2014, 10:19:34 AM
The attached circuit is unfinished, potentially full of value errors because the original was so lo-def; hasn't run yet in LTSpice because the OPT is still unknown
http://www.r-type.org/pdfs/6au6-1.pdf (http://www.r-type.org/pdfs/6au6-1.pdf)
Hello,
Here is where this began some time ago, I have made no progress, I am just as confused now as when I first started. The constant mathematical equations are overwhelming. To completely redesign the circuit and change all of the values and voltages seems a bit of overkill when the amp was functional at one point. All I wanted to do was install a tone stack between the two existing tubes, and it has spiraled out of control to the point that I cannot plug it directly into the wall to see if any progress is being made or not.
I thank you deeply for all your time and effort, but I have to bow out of this project. I am going to restore the circuit to its original specifications and make small changes one at a time, the first being the tone stack in the LTSpice drawing drafted into the original circuit. I am going to replace the 180K resistor with a potentiometer. Change the value of the 560K resistor from power supply to #1 pin on preamp side of the 12AX7.
I think those changes will help. Once again thank you very much for your time and effort but I feel as if
I am in over my head.
Quote from: SpareRibs on January 15, 2014, 02:50:57 PM
Hello,
Here is where this began some time ago, I have made no progress, I am just as confused now as when I first started. The constant mathematical equations are overwhelming. To completely redesign the circuit and change all of the values and voltages seems a bit of overkill when the amp was functional at one point. All I wanted to do was install a tone stack between the two existing tubes, and it has spiraled out of control to the point that I cannot plug it directly into the wall to see if any progress is being made or not.
I thank you deeply for all your time and effort, but I have to bow out of this project. I am going to restore the circuit to its original specifications and make small changes one at a time, the first being the tone stack in the LTSpice drawing drafted into the original circuit. I am going to replace the 180K resistor with a potentiometer. Change the value of the 560K resistor from power supply to #1 pin on preamp side of the 12AX7.
I think those changes will help. Once again thank you very much for your time and effort but I feel as if
I am in over my head.
I commend you for your ability to realize your situation and know when you need to stop. Sometimes just taking a break (a day or even a few months) can help. Good luck with it in whichever direction you end up going :tu:
Quote from: Roly, Reply #9 on: December 26, 2013it would require a fair bit of modification
Quote from: SpareRibsThe constant mathematical equations are overwhelming.
This isn't "mathematics" - it's basic shopkeeping dollars-per-pound
arithmatic; you aren't even doing algebraic transformations, much less using the Operator
j or Calculus, just value substitution and calculator number-crunching. This is the application of Ohm's Law, multiplication and division, and as basic as it gets in electronics.
A couple of significant reasons to modify the original circuit are;
- there is nowhere to insert a tonestack (unless the mic preamp triode is modified to suit a guitar signal)
- the input sensitivity and impedance are seriously wrong for guitar (whichever input is used - you will either get serious front-end overload, and/or excessive loading on the guitar pickup making it dull and thin sounding).
You have been provided with a circuit with ball park values. All you have to do is build it.
Hello,
Given a circuit with ballpark values to build, is a far cry from suggesting a circuit to duplicate. Previously I had explored other options. The Fender Princeton, or Tweed Deluxe tone stacks. There is a circuit with ballpark values that already works and may need some modification.
The value substitution and calculator number crunching is the mathematics I am talking about. It seems to be endless. I don't know where to start what to crunch and to what end or for what purpose. I told you I am not as well versed as you are in electronics.
So as opposed to endless riddles, questions, and dead ends , I have decided to go in another direction. If the solution was as simple as modifying the preamp triode to suit the guitar why was that not the target from the start as opposed to starting from the opposite end of the amp increasing all of the voltage causing
the need to change all of the resistance values and make all sorts of calculations. It all just seems pointless
as the amp was working after a fashion before all of this started.
I have put it back to stock and will start from there adjusting values to get the input similar to one of the Fender schematics.
Once again Thank you for all your time and effort, but I am simply not capable of the disassembling and completely redesigning one component into another
Quote from: SpareRibsSo as opposed to endless riddles, questions, and dead ends , I have decided to go in another direction. If the solution was as simple as modifying the preamp triode to suit the guitar why was that not the target from the start as opposed to starting from the opposite end of the amp increasing all of the voltage causing
the need to change all of the resistance values and make all sorts of calculations. It all just seems pointless
as the amp was working after a fashion before all of this started.
That has been the target since post #8, but 50 years of doing this stuff has taught me that
you have to get the basic starting point right before you start changing things and bamboozle yourself.
The voltage that you posted in post #37 showed there was something basically wrong, see my post #38, and indeed there was a missing ground resulting in crazy voltages.
But by post #43 you still have a problem...
Quote from: RolyBut you measure only 68V, so that obviously doesn't add up.
At this point we are still trying to get a clear idea of what you have with the amp as it originally stands,
before any modification, and trying to answer the question "does the amp as original have any faults/problems?". If we don't first make sure that a) the supply voltages are good, and b) the amp doesn't have any pre-existing faults, then we will get lost chasing our tails. Persisting in trying to start a car that has no fuel in the tank will only get you a flat battery as well.
In fact we had a missing ground that would have stuffed you up big time if it hadn't been identified and fixed before proceeding.
We still seem to have a voltage, 68V on the "preamp" anode,
which can't be right (most likely due to you mistaking which dual triode anode you were probing; this is almost certainly the anode voltage on the Phase Inverter section not the preamp section).
You must remember that you have the amp right under you nose, we don't, and are totally depending on the accuracy of what you post, in this case not the
value of the voltage, but
what voltage you were actually measuring. Posting a voltage from the wrong point is a "fault" as serious as the open ground connection, and must also be sorted out before we can proceed. Do we have a firm foundation on which to build the next step?
Quote from: SpareRibsGiven a circuit with ballpark values to build, is a far cry from suggesting a circuit to duplicate.
Perhaps my "ballpark" is a lot smaller than yours. One variable here is, for example, that the supply voltage in your amp is not
identical to a 5e3, so some
minor tweeking of the
preamp cathode resistor value
might be required for best conditions, but just using the values on the circuit I gave will get some sort of operation
near to what you want.
So let's take a look at the 5e3 circuit you nominated;
(http://www.thevintagesound.com/ffg/schem/deluxe_5e3_schem.gif)
Check out the values around the second stage, the 12AX7 (left), 100k anode load, 1500 ohm cathode resistor, 25uF cathode bypass. Now look at the values around the preamp section of the 12AX7 I drew for you, 12AX7 check, 100k anode load check, 1500 ohm cathode resistor check, 25uF cathode bypass check. In fact the values I nominated as your starting point are
identical to the values Leo fender used in his 12AX7 stage. Yes the grid resistor is different but that is of no significance (other than it may give you a richer tone). That's not just in the ballpark, that's a home run.
Even if we look at the actual preamp using a different dual triode we see that the values are almost the same (the 820 ohm cathode resistor is shared by two triodes so the effective value for a single section will be double, and 1640 ohms is pretty damn close to 1500 ohms).
There are two ways of measuring the current through a valve and its anode and cathode resistors;
a) direct - cut the circuit and insert a milliammeter, or
b) indirect - measure the voltage drop across either resistor and apply Ohm's Law.
We do b) because it is
much easier than cutting and rejoining wires (and because inserting a meter in series can produce other problems that confuse the picture).
There is an awful lot of long-haired maths in electronics, but almost all of it can be avoided, however once you pick up a soldering iron Ohm's Law is part of the turf that
cannot be avoided - pounding some numbers through your calculator is
elemental to what you are doing; trying to avoid Ohm's Law in anything electrical is like trying to drive a car blindfolded.
In fact you have already made
considerable progress by finding and fixing a missing ground, and despite the fact that the amplifier in question is somewhere on the other side of the planet I was able to point you to the problem simply by using Ohm's Law - that's how powerful just one multiplication or division is.
At this point I can only suggest that you Google "Ohms Law" and review pages until you find one that resonates with you, e.g. http://www.allaboutcircuits.com/vol_1/chpt_2/1.html (http://www.allaboutcircuits.com/vol_1/chpt_2/1.html) and http://www.allaboutcircuits.com/vol_1/chpt_2/2.html (http://www.allaboutcircuits.com/vol_1/chpt_2/2.html)
Then you need to do the same with "voltage divider", e.g. http://www.allaboutcircuits.com/vol_1/chpt_6/1.html (http://www.allaboutcircuits.com/vol_1/chpt_6/1.html)
Quote from: https://learn.sparkfun.com/tutorials/voltage-dividersIf learning Ohm's law was like being introduced to the ABC's, learning about voltage dividers would be like learning how to spell cat.
Why voltage dividers? A triode (or transistor) amplifier is basically the same as three resistors in series, the anode resistor, the valve/transistor itself, and the cathode resistor. To get the best out of the stage, indeed to get anything out of the stage, we need to get the voltages on the anode and cathode right, or at least close, and that requires the repeated application of Ohm's Law.
You have a very simple choice here, get comfortable with Ohm's Law, turn on the light, or grope around in the dark and get nowhere fast. You can be your own boss, or just copy what others have done and never understand why things don't work out for you.
Otherwise just copy what I (and Leo) have laid out for you.
Hello,
I am going over this whole thread again to try to come to grips with it. I went back to the very beginning to try to get it all in context. When I gave you the reading of 68 it was off of pin five of the 6AU6 as that is the next in the circuit from the plate of the 12AX7 to the grid of the 6AU6.
I will take a bit of time to reread all of the thread and ponder it in depth. I will get back to this thread after I go through a download of (A Glossary of Common Amplifier Terms), Volume 2 of Basic Electricity by Neville, Nooger & Van Valkenburgh. It covers Ohm's & Kirchhoff's Laws. It is illustrated so it is not just blah, blah, blah, reading. That should help a bit
I guess I let all of this get to me by being impatient, sorry about that. I will try to understand things in
relation to Ohm's law.
I have been in electronics now almost 60 years. I can tell you that without a doubt Ohm's Law is THE most central idea in electronics. And as foreign as it might start out, it really isn;t all that complicated. it simply states that voltage, current, and resistance are related, and how so. I use it every day, in fact I have a cheap little pocket calculator sitting next to me so I can do the arithmetic to apply the Law. Seriously, not a day goes by I don;t calculate some current or voltage or power dissipation in some resistor.
And close on its heels is the idea of voltage divider. Also pretty simple, but they are everywhere. A basic volume control is a voltage divider. The gist of that is that given a couple resistors (or resistances) with a voltage across them, the voltage across each will be proportional to the percentage of each of the total.
In other words, If I have a 40k ohm resistor and a 60k ohm resistor in series, or 100k ohms total, and I put 100v across them, then i will wind up with 60v across the 60k and 40v across the 40k. If I started with only 50v instead of 100v, then the ratio is the same but the voltages are lower, like 30v across the 60k and 20v across the 40k.
Hello,
Yes Enzo, I agree with the fact that Ohm's Law is the basis of any project, and am sorting through all of the changes with that in mind. It takes me longer at times because of some of the abbreviations and terms you guys use. It just becomes frustrating trying to do measurements and doing it in the wrong area. Or worse not knowing where to measure what, to find out why or where the problem is.
I will do the measurements and post them on a drawing from now on to make it clear what I am talking about, that should help everyone involved.
Thank you for your input, it is encouraging to know I am not just plain stupid, I am just not going about it in the proper order.
Hello,
Can someone tell me, after calculating these numbers using Ohm's Law what are they telling me and of what use are they. I just do not know what to be looking for or where to look.
R2-V/284-divided by-.0022=.129090,apparently amperage or current.
R3-V/275-divided by-.056=.001339
Going out to the circuit-V/208-divided by-.027=.007703
Any help or explanation would be greatly appreciated.
Please provide more info. What schematic are those calculations referring to?
What do those numbers represent? As straight math they do not work out.
Hello,
It is the Newcomb Pathfinder schematic, being converted using the LTSpice diagram contained in this thread.
I used a calculator to derive those numbers, but am at a total loss as to what they mean. I divided the voltage into the resistance at each junction in the power supply C2 and C3 then out to the PI side of the 12AX7. of what use they may be I have no idea. I can read the voltage drop across the resistors with my VOM meter. I was trying to find the current, using Ohm's Law. It is all very confusing.
Thanks for your response.
I wondered what we were still working on also, why not just put up another link so we don;t have to go sort back through four pages to find the schematic.
If you have 250v at one end of a resistor and 230v at the other, then 20v was dropped across it. or just measure the 20v end to end as it sits. Ohm's Law says I = V/R. SO if we have for example a 4700 ohm resistor (4.7k), then 20/4700 = .004. The units of Ohm's Law are amperes, volts and ohms. SO that 0.004 is in amperes. 0.004A is the same thing as 4milliamps, 4ma. So in my made up example, if I have 20v across a 4700 ohm resistor then 4ma current is glowing through it.
I don;t know what all the numbers you posted are.
Ohm's Law works for any combination of two of the three components. SO if I have a 6800 ohm resistor and I know it has 14ma flowing through it, I can figure V = IxR = 0.014 x 6800 = 95v. I'd have 95v across that resistor.
A word about numbers. Your calculator might come up with a number like 0.007703, but your figures cannot be that precise. We have what we call significant digits in a number. If I have 45v, that has two significant digits. If I have occasion to divide 45 by 7, my calculator will tell me it is 6.4285714. But the most I have is two significant digits, so I cannot use more than 6.4 of my answer. If my original measurements were 45.137v, then I would have 5 significant digits, and I can make more precise calculations.
But really, this is a guitar amp, approximate is more than enough. If you have 0.001339 amperes, that is 1ma for any discussion we might have. The extra 339 microamps will not make any difference.
Hello,
Thank you that really helps a lot explaining how to derive the calculations. I have made all of the modifications and all of the readings are pretty close as the transformer does not put out the same amount of volts as the LTSice drawing. I am going by the Newcomb schematic for the incoming voltage and resistor readings between C2 and C3.
I will open a new thread if I don't get results soon. I am going to take the readings one more time and if it
all checks out I am plugging it in directly to the wall.
Thanks again sorry to be such a PITA.
Ok, here is a repost of the schematic.
As Enzo mentioned, the voltage you use should be what is measured across the resistor. And somehow you are going the wrong way with your decimals. A 270K resistor is 270,000 not .027 or anything, for calculations use values in ohms.
So, according to the values on the schematic:
R2 is 2200 ohms. There is 312V on one end, 303V on the other. That means 9V across R2. 9V divided by 2200 = .004 That is the current in amps, or can be called 4mA (milliamps). Do you know the metric system? It uses a lot of the same abbreviations and makes electronics easier to understand.
Anyway, we have 4mA of current flowing through R2.
R3 has 71V across it. It is a 57K resistor. So 71 divided by 57000 = .001A or 1mA
So what do these numbers do for me? Well, there is a difference of 3mA. So some of the current that was flowing through R2 is not flowing through R3. Where did that 3mA go? Look at the original drawing and you will see that there is a line from R2/R3 junction to the power tubes. This is the screen grid. So that is where the extra 3mA went. The spice drawing is missing that line so it is a typo.
So in this case, those numbers helped me find a typo, but generally we use them to see if tubes are drawing proper current etc.
If you like, figure out the current through R10. Then, if I tell you that plate current of a tube is pretty much the same as cathode current, you should be able to figure out what the voltage should be across R11.
Hello,
THANK YOU. You have explained more now than I have been able to glean from any of the 56 previous posts. That brings it all together. I could not understand how to find the amperage carried forward from one resistor to the next. All of the calculations I made on the calculator didn't come up with any useful answers.
Now I can go back over the numbers and it will make more sense. Thanks again.
Hey SpareRib,
You may find something here will help to get your head around how it all works.
http://www.valvewizard.co.uk/
Comes with pics and not overloaded with maths. 8)
Phil.